Haskell Pattern Matching (beginner)

I have to implement a small programm in Haskell that increments/decrements a result by what in the console line is. For example if we have -a in the console the results must be 0, if -b the result must be incremented with 6 and so on. I have to do this with pattern matching.

I haven't used Haskell until now and I find it pretty hard to understand. I have this to start with:

import System.Environment
main = getArgs >>= print . (foldr apply 0) . reverse
apply :: String -> Integer -> Integer

I don't understand what in the main is. What does it make and the reverse from end, what does it do? As I've read on the internet the getArgs function gives me the values from the console line. But how can I use them? Are there are equivalent functions like for/while in Haskell?

Also, if you have some examples or maybe could help me, I will be very thankful.

Thanks!